The invention relates to high definition television (HDTV) and to spread-spectrum (SS) communication systems.
In U.S. Pat. No. 5,127,021 (the '021 patent), incorporated herein by reference, a SS TV transmission system is described having certain advantages over known systems. These include the ability to establish the relative SNR of recovered components of different spatial frequency, the ability to optimize the quality of the reconstructed picture at each level of receiver CNR, and the ability to transmit more frequency components than would otherwise be possible within the allotted bandwidth by exchanging the total bandwidth of the components for SNR. (Note that the term CNR as used in this specification refers to carrier-to-noise ratio of radio-frequency signals, and the term SNR refers to the signal-to-noise ratio of input and output video signals.) Although it is a true SS system, the total bandwidth required to transmit a true HDTV image is no more than 6 MHz. This is the result of first dividing the original wideband signal into a large number of narrow-band streams, then spreading each such narrow-band stream to 6 MHz by direct-sequence SS, and finally adding up the coded and spread streams.
In addition to the problems of terrestrial broadcasting discussed in U.S. Pat. No. 5,127,021, reflections from buildings, mountains, and other objects nearly always give rise to echoes, evidenced in the received image as "ghosts" due to multipath transmission. Such ghosts are one of the main aspects of image degradation as seen at typical reception sites. Spread-spectrum transmission is a well known method of dealing with ghosts, although it has so far not been applied to TV since, in general, it is not permissible to expand the bandwidth as is done in conventional SS systems. The theory of SS ghost suppression is given in a paper by Turin. (G. L. Turin, "Introduction to Spread-Spectrum Antimultipath Techniques," Proc. IEEE, 68, 3, March, 1980, pp 328-353.)